1. Field of the Invention
The present invention relates generally to all-channel television tuning systems and more particularly to an electronic tuning system having a time sequenced presettable counting system for determining the system tuning condition by counting the frequency of a tunable element such as a local heterodyne oscillator.
1. Description of the Prior Art
U.S. Pat. No. 3,753,119 issued Aug. 14, 1973, to Close shows a digital indicator for periodic display of the signal frequency to which a receiver is tuned. A counter is preset and supplied with a submultiple of the corresponding receiver local oscillator frequency for a prescribed time interval such that the counter output denotes the actual received frequency (or corresponding television channel number depending upon manual selection of the appropriate frequency band). The Close patent avoids the complex counting problem encountered in the VHF television spectrum, which comprises three irregularly spaced frequency bands, by directly inserting VHF television channel numbers into the counter through "hard wired" circuits for display.
A system employing channel number information to produce tuning is described in U.S. Pat. No. 3,778,736 issued Dec. 11, 1973, to Sakamoto, and shows a pair of memory circuits connected to a matrix network for directing binary coded channel number signals to a set of logic gates to selectively connect one of a set of voltage dividing networks to appropriate variable capacitance diodes (varactors) for tuning. The matrix converts the channel number into an appropriate matrix position and provides numbered positions from 1 to 62, with the first 12 positions reserved for VHF channels.
The only band discrimination is that for switching the VHF tuner between a low and high band. The outputs of the first three voltage positions (corresponding to channel numbers 2, 3 and 4) are coupled to a NOR gate for operating the tuner in its low band mode. The next seven voltage positions, numbered 4 to 12 (channel numbers 5 to 13) have their outputs coupled to a NAND gate for operating the tuner in its high band mode.
In the system disclosed in the above mentioned Tanaka application a modular counter is reset to count the local oscillator frequency on a sample basis and supply channel number information to a comparator, to the other input of which is supplied the desired channel number information. The comparator output is used to control a ramp drive voltage source for changing the tuner oscillator frequency. The counters are supplied individual presets for the four frequency bands embracing all allocated FCC channels. The band decoder utilized in this inventive combination distinguishes four groups of channel numbers 2, 3 and 4; 5 and 6; 7-13 and 14-83. Appropriate presets are loaded into the counters in accordance with the frequency band in which the television signal corresponding to the selected channel number lies.